In both procaryotic and eucaryotic cells, molecular chaperone proteins catalyze disulfide bond exchange and assist in the proper folding of newly synthesized proteins. This observation has led to a large number of studies and proposed uses for these quality control proteins. For example, increasing pDI (protein disulfide isomerase) activity in bacterial, yeast and insect cell expression systems can have beneficial effects on protein solubility and folding and, in some cases, can lead to an increase in the secretion of heterologous proteins (1-7). In addition, other studies have shown that the molecular chaperones immunoglobulin heavy chain binding protein (BiP, also referred to as glucose regulated protein) and human heat shock protein 70 (Hsp 70) have a beneficial effect on recombinant protein expression in insect cell systems (5, 8-12).
Molecular chaperones have not had the same level of success on recombinant protein expression and secretion in mammalian cell systems. For example, overexpression of the pDI chaperone in Chinese hamster ovary (CHO) cells not only had no effect on the secretion levels of IL-15, but also caused a decrease in secretion, and an increase in cellular retention of a tumor necrosis factor receptor-Fc fusion protein (TNFR:Fc) (13). Other studies have shown that overexpression of the BiP chaperone in mammalian cells can lead to increased cellular retention and decreased secretion of recombinant proteins (14-15 and U.S. Pat. No. 4,912,040). The regulatory mechanisms involved in protein processing within the mammalian cell are complex, and probably involve the cooperation of many of these chaperone proteins. Therefore, one cannot predict whether a particular chaperone will lead to an increase in the production of a certain recombinant protein.
Because of the contradictory teaching in the field, the effect of chaperone proteins on the production of a secreted recombinant protein product is not understood and appreciated. U.S. Pat. No. 6,451,597 (the '597 patent) describes a method for enhanced production of viral particles, and speculates on the effect of chaperones on improving yield of a recombinant protein in eukaryotic cells. However, no actual expression of a recombinant protein is disclosed. However, other studies had found that over-expression of chaperones in eukaryotic cell lines either had no effect on product yields or had reduced secretion of recombinant proteins (14, 15). See also U.S. Pat. No. 4,912,040. In light of the contradictory teaching in the field, the '597 patent does not enable one of skill in the art to use chaperones to improve the production and secretion of a recombinant protein in eukaryotic cells. The state of art does not teach one to predict what effect a particular chaperone will have in the production and secretion of a given recombinant protein in cell culture models such as those described herein. The applicants were therefore surprised to find that when the chaperones described in this study were transfected into mammalian cell lines expressing a secreted, recombinant protein, the resultant effect was an overall increase in the production of the secreted protein.